While few disagree with the need to increase the number of trees lining the nation’s cities and streets, it is important that such advances proceed deliberately, in accordance with well-conceived plans. While it is true that most trees provide tangible, quantifiable benefits, they also require resources to maintain.
Selecting the wrong trees for a given location not only fails to maximize the potential benefits — which a better species could provide — but it increases the likelihood of negative consequences. If a city plants labor-intensive trees, maintenance costs chip away at the realized savings; likewise, a city that plants trees with invasive root systems must allocate sufficient funds to repair damaged sidewalks and other components of the infrastructure.
The ideal trees for a given location vary greatly, so policy makers are wise to consult with an industry expert before designing a tree plan and selecting the species, which will make up the planting. Different species thrive in different hardiness zones and under different hydrological conditions.
Nevertheless, most street trees have a few common characteristics. None of these traits is singly essential, but the more of the characteristics present in a tree, the more effective it will perform as a street tree.
Reasonable Roots
Tree roots are opportunistic, and they can exacerbate any faults or flaws present in the surrounding hardscape. Given enough time, even relatively modest-sized trees can raise sidewalks or compromise foundations. Accordingly, it is important to plant trees that have manageable root systems. Avoid installing species that produce large surface roots, such as Blackwood acacia (Acacia melanoxylon), Norway maples (Acer platanoides) and camphor trees (Cinnamomumcamphora) in areas adjacent to concrete features. Instead, opt for species that are unlikely to cause such problems, such as Pacific wax myrtles (Myrica californica), Pacific dogwoods (Cornus nuttallii) and madrones (Arbutus menziesii).
Suitable Stature
One of the most important considerations regarding street tree selection is the mature height and spread of the crown. Trees that outgrow their allotted space may end up rubbing against buildings, blocking the right-of-way and growing dangerously close to utility lines. The pruning necessary to mitigate these problems increases maintenance costs, thus reducing the economic impact of the tree. Several small species, including western redbuds (Cercis occidentalis) and Macnab cypresses (Hesperocyparis macnabiana), have many of the attributes one would want in a street tree, while usually remaining under about 25 feet or so in height. Of course, there is nothing wrong with planting tall species in areas without overhead obstructions; however, the crown’s spread – horizontal growth – is still an important consideration for street plantings. Trees with a columnar growth habit are often helpful in this regard, such as Columnar Sargent Cherries (Prunus sargentii ‘Columnaris’), which grow up to 35 feet high, but usually have a crown spread of less than 15 feet.
Limited Labor Liabilities
While the cost benefit ratio of many street trees is a net positive, selecting species that require frequent maintenance increases the odds of ending up in the black. For example, wild-type mulberries (Morus spp.) and sweetgums (Liquidambar styraciflua) produce copious fruit, which often necessitates frequent attention. Either select cultivars that produce no fruit – such as “Rotundiloba” sweetgums (Liquidambar styraciflua ‘Rotundiloba’) – or select lower-maintenance species entirely. Additionally, drought tolerance is an important consideration for all southern California tree installations. Some species with exceptional drought tolerance include scrub oaks (Quercus dumosa) and “Shademaster” honeylocusts (Gleditsia triacanthos var. inermis ‘Shademaster’), which although unsuitable for use near sidewalks, require little maintenance and allow enough light penetration that grass grows well underneath them.
Superb Solar Solutions
Some deciduous trees provide two different solar-related benefits at opposite ends of the calendar. During the summer, they provide dense shade, but in the winter, when their leaves carpet the ground below, these trees allow the warm rays of the sun to penetrate to ground level. The maidenhair tree (Gingko biloba) – especially the Autumn Gold cultivar (Gingko biloba ‘Autumn Gold’) – provides a great example of this. In fact, gingko trees are not only “solar friendly” and capable of surviving the indignities of urban living, their leaves turn marvelously yellow in the fall. Unlike normal (female) maidenhair trees, the Autumn Gold cultivar does not produce the smelly fruit, characteristic of the species.
Pollution-Proof
Whether by runoff water, soil contamination or through the air, street trees exist under the constant assault of pollutants. These substances stress many trees, causing a reduction in vigor, often preventing them from thriving. Ideally, trees planted along streets should be very tolerant of pollution – especially air pollution, which constantly bathes the trees in harmful chemicals. Hedge maples (Acer campestre) are particularly well suited for urban environments, although they thrive best in moist, cool locations. Hedge maples not only tolerate pollution well, but they rarely produce roots that cause problems, nor do they struggle with compacted soils. Additionally, hedge maple leaves are often brilliant gold in the fall, making them a gorgeous addition to any street.